1. Field of the Invention
The present invention relates to electropolishing devices and methods, and more particularly to electropolishing devices for polishing stents and other cylindrical medical devices.
2. Summary of the Related Art
In the past, permanent or biodegradable devices have been developed for implantation within a body lumen or blood vessel to maintain patency of the lumen. Once expanded within the blood vessel, these devices, called stents, become encapsulated within the body tissue and provide a permanent or biodegradable support for the vessel wall.
Known stent designs include thin-walled metal cylinders with axial slots formed around the circumference (U.S. Pat. Nos. 4,733,665; 4,739,762; and 4,776,337). Known construction materials for use in stents include polymers, organic fabrics and biocompatible metals, such as, stainless steel, gold, silver, tantalum, titanium, cobalt chromium alloys, and shape memory alloys, such as Nitinol.
Stents are often formed by cutting a pattern into a tube, such as a stainless steel or cobalt chromium alloy tubes with a laser. However, laser cutting and other metal forming operations tend to leave burrs and sharp edges which are undesirable on implantable devices. The stents will be in contact with tissue and burrs and sharp edges can cause tissue irritation. Tissue irritation caused by burrs and rough surfaces on stents can be one of the causes of restenosis or the re-occlusion of a blood vessel after balloon angioplasty and stenting. In addition, when sharp edges, burrs, or other marks are on the inside of stents they can cause disturbance in the blood flow through a blood vessel which can cause the blood to clot. Electropolishing is a process used for providing a smooth surface on metal parts including implantable metallic devices, such as stents.
Electropolishing involves immersing a metallic object into an electrolyte solution in the presence of an electric current which causes protrusions on the metal object to be dissolved. The metallic object, such as the stent, is positioned in contact with an anode or positive electrode, while a cathode or negative electrode is positioned in the electrolyte solution nearby. As the electric current passes through the metallic object and electrolyte solution, from the anode to the cathode, metal is removed from the object. However, the metal is removed faster at the locations of protruding portions of the metal due to a concentration of the current at these locations. The dissolving of the protrusions faster than the remaining metal surfaces results in a very smooth and polished surface.
U.S. Pat. Nos. 6,299,755; 6,375,826; 6,599,415; and 6,679,980 disclose systems for electropolishing stents. Although electropolishing is an effective method for obtaining smooth surfaces on stents, electropolishing processes are very sensitive. Repeatable and even polishing is difficult to achieve. The uniformity of the polished surface depends on factors including on the uniformity of the current density across the surfaces. The current density is particularly uneven at the contact points where the anode is in contact with the metal surface. The high current density at the contact point result in particularly aggressive polishing around the contact points. In addition, the metal contact member, which may be a clip, a wire, or a roller, can mask the current and cause a high point. These contact points result in contact marks on the polished products which are undesirable.
Accordingly, an electropolishing fixture which minimized current density differences and substantially eliminates contact marks on the products is needed. In addition, it would be desirable to provide overall even electropolishing.